Vacuum tube amplifier



March 8, 1932. J. o. M NALLY VACUUM TUBE AMPLIFIER Filed July 9, 1930FIG. I.

nun WWII" I FIG. 2.

M/VE/VTOR J. 0. M /VALL V ATTORNEY Patented '8, 1932 UNITED/STATESPATENT OFFICE JAMES o. ncnmx, or mrnnwoon,

NEW JERSEY, ASS IGNOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, OFNEW YORK, N. Y A. CORPORATION OF NEW tom: 7

. vacuum: ma mm].

Application" filed July 9,

This invention relates to vacuum tube amplifiers, and more especially toamplifiers m which the vacuum tubes are arranged in pushpull relation.

5 It is well known that in push-pull amplifiers where either high poweror ,high quality or both are involved the two tubes should be as'nearlyalike in their characteristics as possible and that if they'a're notpractically 10 identical much trouble is experienced due to undesirableoscillations which may be set up,

I or to a considerable lowering of the quality -of the output of theamplifier. It is also well known especially in high power or compara- 15tively high platecurrent installations that if for any reason one of thetubes becomes inactive the second tube has impressed upon it such alarge increase of plate current that the second tube may be damaged.

29 "An object of this invention is to improve the operation of push-pullvacuum tube amplifiers.

Another object of the invention is to have a high degree of balancebetween the two sides of'a push-pull amplifier without exercisingextreme care in choosing the tubes.

A further object is to afiord protect on to one of the tubes in apush-pull arrangement p if the second tube should he accidentally or sointentionally out out of the circuit.

In one embodiment of the invention a grid biasresistance unit isinserted in the plate circuit of-each tube between the cathode andthe-negative side of theplate source. With current through one of thetubes above its normal value will change the potential relations betweenthe grid, cathode and anode of thetube in a manner to oppose theincrease in the plate current. With the plate currents heldapproximately equal, less expensive output transformers may be usedsince the condition of. direct current saturation in one winding therimprovement is noted due to the inherent 'further elimination of secondharmonics.

The invention will be better understood from the following descriptionin connection W with the attached drawings, Fig. 1 of which sucharrangement a slow increase of plate of the core by excess of currentmay be disregarded. A furbe 1980. Serial No. 486,621.

or cathodes 5, 5 and anodes 6, 6. The midpomt of winding 2 is connectedthrough resistance 7 to the negative terminal 13 of the anode currentsuppl which is usually made the ground of the amplifier.

The filaments 5, 5 are heated by alternating current derived fromsecondary windmgs 8, 8' of a power transformer, the primary winding 0which may be connected to any convenient source. natjng current and theprimary winding have been omitted from the drawings since thearrangement of this apparatus is well known. The mid-point of eachwinding 8, 8 is connected throughleads 11 11'- and resistances itsoutside terminals connected The source of alter- 9, 9 to the negativeterminal 13 of the anode current source.

winding 10 is connected to the positive terminal 14 of anodecurrentsource in theusual manner.

In push-pull amplifiers using alternatin current for filament heatingheretofore it has been customary to connect the filaments in parallel toone winding ofthe power transformer; A connection from'the mid-point ofthe winding containing a resistance shunted.

by. a condenser to the mid-point of the transformer secondary completedthe input circuit for both space discharge devices. With sucl; anarrangement the grids of both devices hav 1 en maintained at thesamepotential.

' lathe-present arrangement as pointed out, above, each filament has aseparate winding and separate resistances are used in each filamentreturn. v

' In circuits involving the use of a resistance sistance capacltyfiltering is the grid circuit,

' er transformer,

in the plate circuit to establish grid bias, re-

provided in the grid circuits. The filter for grid 4 comprisesresistance 7 and condenser 12. That for grid 4 comprises resistance 7and capacity 12'. Resistance 7 is large in comparison with 9 or 9' andis efiective to ether with the condensers in preventing rapi variationsin space current due either to the signal. or to hum or noise componentsfrom reaching the ids. Resistances. 9 and 9' although forming part ofthe resistance path in parallel to the condensers do not really part ofthe filter. It is because 0 the ripple likely to occur in resistances 9and 9' that the filters are necessary.

The operation of push-pull amplifiers is well understood and hence willnotbe repeated here except as to the direct current anode circuit. Theanode direct current path for device 3 may be traced from the positiveterminal 14 of the anode current source through the up r part of winding10 to anode 6, and

ament or cathode 5, winding 8 of powlead 11, resistance 9 to thenegative terminal 13 of the source. A similar path including the lowerhalf of winding 10 and resistance 9' may be traced for device 3'.

An IR drop occursin resistance 9 (or 9 by the ge of anode currenttherethrou and this drop determines the potential of t e from filamentabove that of the negative terminal of the anode current source orground. Since practically no direct current .flows through the IR dropthrough rebe disregarded and the grids at the potential of the negaanodecurrent source or sistance 7 may considered to be tive terminal of theound.

However, if the anode current varies, the

IR drop through resistance 9 (or 9) will vary and the potential of thefilament will also vary. If the anode current should increase, the IRdrop through the resistance will increase and the potential of thefilament becomes more ositive. The effect is the same as if the gribecame more negative and the anode less positive. That is, there wouldbe a reduction in anode current flow. The opte action occurs if shouldfall below its normal value. In such casethe filament becomes lesspositive or, puttingit the other way, the grid becomes less 11 ative andthe anode more positive.

is control action occurs no matter what the cause of the change may be.In the case ofa single resistance to control the bias of both tubes, ifone tube-should be removed from the circuit there is a tendency for thecurrent through the other tube to greatly inwhich increase if maintainedwould crease probably damage the-tube. The same is true.

if by accident anode current through one tube is cut ofi. In any case,the arrange-.

form a for tube 23 may be fore is to be limited and an anode, an anodecircuit the anode currentment functions to bring the anode current ineach tube back to its normal value.

The most important function, however, of this arrangement is to maintainbalance between two tubes which do not have identical characteristics.In such a case, the resistances 9 and 9 may be given such values as toprovide a normal grid bias for each tube, in which case the currentthrough both tubes are approximatel equal and hence there is practicallyno un alance.

In Fig. 2 equi-potential cathode or heater t pe tubes are used. In thiscase the heating ements 3 3 and 33' are connected in paralle to a singlewinding 28 of the power transformer. Whereas the heating elements areshown connected in parallel the, invention should not be so limited as aseries connection will function as well and in some cases is preferred.The cathodes 25 and 25 are directly connected to resistances 29 and 29',respectively. The anode direct current path traced from the positiveterminal 44 of the 'anode current source through the upper half ofwinding 30, anode 26, cathode 25, resistance 29 to the negative terminal43 of the source. Variations in anode current flow vary the potential ofthe cathode with the same result as described in detail in connectionwith the circuit of Fig.) 1.

It is to be understood that whereas ut two embodiments of the inventionrelating to amplifiers have been described, the invention is alsoapplicable to oscillators and thereonly by the scope of the a pendedclaims.

at is claimed is:

1. A vacuum tube amplifier; comprising vacuum tubes arranged inpush-pull relation, each tube containing a control gr1d,a cathode foreach tube and means comprising an individual impedance in each anodecircuit to control the potential difference between each grid and itsassociated cathode.

2. A vacuum tube amplifier vacuum tubes arranged in push-pull relation,each tube containinga control grid, a cathode and an anode an anodecircuit for each tube and an individual resistance in each anode circuitby means of which the tential of the grid is controlled relative to epotential of the cathode.

3. In an electric circuit, vacuum tubes arranged inpushull relation,eachtube inclu ing a contro electrode, a cathode and an anode, means toinsure equal anode direct current How in each tube comprising a separateresistance element in each anode circuit, the IR drop through which, dueto the anode current, controls the potential differences beranged inpush-pull relation, each tube incomprising cluding a control electrode,a cathode and an anode, means to insure equal anode 'direct' currentflow in each tube comprising a separate resistance element in each anodecircuit connected between the corresponding cathode of each tube and thenegative terminal of the anode current source, the IR drop throughwhich, due to the anod'ecurrent, controls potential difierences betweenthe electrodes of the corresponding vacuum tube.

5. In an electric circuit, vacuum tubes arranged in push-pull relation,each tube including a control electrode, a cathode and an anode, acommon source of anode current, independent cathode energizing sources,

.a connection. including a resistance from each of said cathodes to thenegative terminal of said source of current, and connections from thegrids to said terminal.

.6. In an electric circuit, vacuum tubes arranged in push-pull relation,each tube including a control electrode, a cathode andv an anode, aconnection between each cathode and its associated control electrode, aseparate resistance included in each said connection, a resistancecommon to both said connections and a separate condenser in shunt tothat portion of each connection containing said resistances.

I In witness whereof, I hereunto subscribe my name this 7th day of July,1930.

JAMES O. McNALLY.

